Hemophilia A is a congenital bleeding disorder caused by genetic mutations affecting a plasma protein, termed factor VIII (fVIII), whose function is to facilitate blood clotting. State of the art treatment for hemophilia A consists of frequent intravenous infusions of fVIII containing products. The current limitations to treating hemophilia are 1) the cost of fVIII products, 3) the development of immune responses against fVIII that block treatment efficacy, 3) morbidity due to joint disease resulting from repeated bleeding into individual joints and 4) the limitation of treatment to 30% of the world population. Due to the limited amount of fVIII needed to provide clinical benefit to the patient, hemophilia A is an attractive disease target for gene therapy, and three phase 1 clinical trials have been conducted. The outcome of these trials has been disappointing due to the extremely low, non-therapeutic levels of fVIII produced in each of the gene therapy strategies. We recently showed that a modified porcine fVIII transgene, designated BDDpfVIII, facilitates very high-level protein expression, and we demonstrated proof-of-concept that this transgene functions extremely efficiently in a mouse model of hemophilia A following transplantation of genetically-modified hematopoietic stem cells (HSCs). Specifically, we have shown that the expression of BDDpfVIII is superior to other bioengineered human fVIII expression constructs and that genetic modification and transplantation of HSCs results in curative fVIII levels. Additionally, curative fVIII activity levels are achieved after transplantation of BDDpfVIII-transduced HSCs following low-toxicity pre-transplantation conditioning with targeted immunosuppression, even in the context of pre-existing anti-human fVIII inhibitors. Therefore, we have overcome the major hurdle of low-level expression using a transgene that encodes a protein that has been used successfully in patients with hemophilia A. In the current application, we propose to more fully characterize the use of the high-expression construct and further our understanding of the critical parameters involved with this novel gene therapy strategy and study the biology of non-physiological BDDpfVIII expression in hematopoietic (blood) cells. To advance our studies toward clinical significance, we propose to 1) test clinically relevant HSC transplant conditioning regimens that more closely resemble those used routinely in human bone marrow transplant protocols and 2) test recombinant lentiviral vectors that have been demonstrated to display a reduction of insertional mutagenesis compared to oncoretroviruses. Finally, the optimized lentiviral vector(s) encoding BDDpfVIII will be tested for the ability to genetically modify human HSCs.